Subsurface safety valves are well known in the oil and gas industry and act as a failsafe to prevent the uncontrolled release of subterranean reservoir fluids in the event of a worst-case scenario surface disaster. Typical subsurface safety valves comprise flapper-type valves that are opened and closed with the help of a flow tube moving telescopically within a production tubular or tubing string. The flow tube is often controlled hydraulically from the well surface and is forced into its open position using a piston and rod assembly that may be hydraulically charged via a control line linked directly to a hydraulic manifold or pressure control system at the well surface. When sufficient hydraulic pressure is conveyed to the subsurface safety valve via the control line, the piston and rod assembly urges the flow tube downwards, which compresses a spring and simultaneously pushes the flapper to its open position. When the hydraulic pressure is removed from the control line, either manually or in the event of a surface disaster, the spring pushes the flow tube back upwards and out of engagement with the flapper, which allows a torsion spring to move the flapper back to its closed position.
As the setting depths of hydraulically-actuated subsurface safety valves continue to increase, the robustness or thickness of associated flappers also increases to accommodate elevated downhole pressures and temperatures. Thicker flappers, however, are more difficult to fully close using conventional torsion springs, and, if a flapper fails to fully close, potentially hazardous reservoir fluids may escape into the surrounding environment via the partially closed tubing string.